An integrated electronic circuit comprises, on the one hand, elements produced by ion implantation in a semiconductor substrate, such as diodes or transistors, and, on the other hand, metallic elements distributed within layers of insulating materials that are placed on top of the substrate. Six to eight layers of insulating materials may be superposed. In general, the metallic elements are interconnects that electrically connect the elements produced by ion implantation in the substrate. They may also be capacitors, coils, inductors or even antennas. They are generally produced using the damascene method or using its variant, dual damascene, known to those skilled in the art.
The electrical behaviour of the metallic elements may be disturbed by electrostatic or electromagnetic coupling effects that appear between these metallic elements when they are placed so as to be particularly close to one another. These coupling effects are stronger the higher the level of integration of the electronic circuit. Those of these coupling effects that are electrostatic in nature may be likened to parasitic capacitances occurring between neighbouring parts of different metallic elements. This is especially the case when the integrated electronic circuit has a pitch of less than 5 microns.
It is known to reduce the electrostatic coupling effects by introducing, between said metallic elements, an insulating material of low dielectric permittivity in the layers within which the metallic elements lie. Thus, layers of polymers and layers of porous materials have been used, these having relative dielectric permittivities of around 2.2 to 2.5. However, the resulting reduction in electrostatic coupling is limited, and insufficient vis-à-vis the currently envisaged level of integration of the electronic circuits.
It is also known to introduce air gaps between neighbouring metallic elements in order to reduce these coupling effects. To do this, after a layer of insulating material has been produced and after metallic elements have been produced in this layer, a hard mask is deposited on the layer, which mask includes apertures in line with certain of the spaces that separate the metallic elements. The insulating material is then etched via said apertures, in order to form trenches. An upper layer is then deposited on the layer of insulating material and in the trenches, so as to close the trenches without filling them. The air gaps thus formed are generally of small dimensions. In particular, parts of insulating material remain between the metallic elements. In addition, such a process requires a specific lithography mask, of high definition and therefore expensive. It also requires precise alignment of this lithography mask relative to the substrate of the integrated electronic circuit.
It is also known to produce metallic elements within a layer of sacrificial material, to continue the production of the circuit by depositing additional layers of insulating material above the layer of sacrificial material, which may also incorporate metallic elements, and then to remove the layer of sacrificial material. The additional layers and the metallic elements that they incorporate are then connected to the substrate of the electronic circuit only via the metallic elements initially produced in the layer of sacrificial material. The integrated electronic circuit then lacks cohesion or mechanical robustness.